1. Field of the Invention
This invention is directed to wellbore drilling top drive systems; such systems including apparatus for sensing deflection of a top drive main shaft; and methods of their use.
2. Description of Related Art
The prior art discloses a variety of top drive systems; for example, and not by way of limitation, the following U.S. patents present exemplary top drive systems and components thereof: U.S. Pat. Nos. 4,458,768; 4,807,890; 4,984,641; 5,433,279; 6,276,450; 4,813,493; 6,705,405; 4,800,968; 4,878,546; 4,872,577; 4,753,300; 6,007,105; 6,536,520; 6,679,333; 6,923,254—all these patents incorporated fully herein for all purposes.
Certain typical prior art top drive drilling systems have a derrick with a top drive which supports and rotates tubulars, e.g., drill pipe. The top drive is supported from a travelling block beneath a crown block. A drawworks on a rig floor raises and lowers the top drive. In many cases, a top drive is secured to a dolly that moves on a guide track in the derrick.
A top drive has a main drive shaft that is rotated by one or more motors. This main drive shaft supports significant weights, including, during certain operations, the weight of a drill string. For effective and efficient operations, it is important that the top drive main shaft remain aligned with a load supported on the top drive main shaft and/or with a well center of a well above which the top drive is positioned. Misalignment can result from incorrect positioning of dolly guide tracks or incorrectly positioning a top drive on a dolly, either laterally or at an angle to a well center line. Misalignment can also result if a dolly retract system does not position the top drive over well center.
In the past, efforts to maintain alignment of a top drive main shaft have included various mechanical position or attitude adjustment apparatuses and arrangements of hydraulic cylinders to relieve bending loads caused by shaft misalignment. In the past, due to the relative high stiffness of a top drive main shaft, it has not been obvious to use a sensor to detect top drive main shaft deflection. This was also not obvious because the main shafts are so stiff that detecting damaging bending was beyond economical sensor resolution.